Aircraft main engines not only provide propulsion for the aircraft, but in many instances may also be used to drive various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical, pneumatic, and/or hydraulic power. However, when an aircraft is on the ground, its main engines may not be operating. Moreover, in some instances the main engines may not be capable of supplying power. Thus, many aircraft include one or more auxiliary power units (APUs) to supplement the main propulsion engines in providing electrical and/or pneumatic power. An APU may additionally be used to start the main propulsion engines.
An APU is, in most instances, a gas turbine engine that includes a combustor, a power turbine, and a compressor. During operation of the APU, compressor draws in ambient air, compresses it, and supplies compressed air to the combustor. The combustor receives fuel from a fuel source and the compressed air from the compressor, and supplies high energy compressed air to the power turbine, causing it to rotate. The power turbine includes a shaft that may be used to drive the compressor. In some instances, an APU may additionally include a starter-generator, which may either drive the turbine or be driven by the turbine, via the turbine output shaft. The starter-generator may be operated as either a motor or a generator. When operating as a motor, the starter-generator may be used to drive the turbine while the APU is being started, and when operating as a generator, it may be driven by the turbine and used to supply electrical power to the aircraft power distribution system.
In many instances, an APU may include some type of gearing between the turbine output shaft and the starter-generator. This can increase the overall size and weight of the APU. Some APUs have been proposed that do not include such gearing, and instead include a single shaft that interconnects a power turbine, a compressor, a cooling turbine, and a starter-generator. However, such APU configurations may also suffer certain drawbacks. For example, extensive design and high manufacturing tolerances may be needed to avoid unwanted shaft vibrations, which can increase overall costs. Moreover, such APUs have thus far been implemented using multi-stage compressors and/or turbines, which can increase the overall size, weight, and cost of the APU.
Many APUs may also include starter-generator control units that are disposed external to the APU, and thus occupy space within the aircraft fuselage. In addition, many APUs may include one or more components that rely on a flow of lubricant. Thus, the APU may be designed and implemented to include one or more interfaces to external lubrication systems, which can increase overall size, weight, and cost.
Hence, there is a need for an APU that can be implemented without the need for extensive design and/or high manufacturing tolerances, and/or without relatively bulky multi-stage compressors and/or actuators, and/or without external control units, and/or that does not rely on a flow of lubricant. The present invention addresses one or more of these needs.